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[ The PC Guide | Systems and Components Reference Guide | Motherboard and System Devices | System Resources | Interrupts (IRQs) | Interrupt Function and Operation ]

Interrupt Priority

The PC processes device interrupts according to their priority level. This is a function of which interrupt line they use to enter the interrupt controller. For this reason, the priority levels are directly tied to the interrupt number:

  • On an old PC/XT, the priority of the interrupts is 0, 1, 2, 3, 4, 5, 6, 7.
  • On a modern machine, it's slightly more complicated (what else is new). Recall that the second set of eight interrupts is piped through the IRQ2 channel on the first interrupt controller. This means that the first controller views any of these interrupts as being at the priority level of its "IRQ2". The result of this is that the priorities become 0, 1, (8, 9, 10, 11, 12, 13, 14, 15), 3, 4, 5, 6, 7. IRQs 8 to 15 take the place of IRQ2.

In any event, the priority level of the IRQs doesn't make much of a difference in the performance of the machine, so it isn't something you're going to want to worry about too much. If you are a real performance freak, higher-priority IRQs may improve the performance of the devices that use them slightly. If you could actually notice this in any way other than examining the system under the microscope of a benchmark suite, I'd be pretty surprised...

Next: Non-Maskable Interrupts (NMI)

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